Various lubricating oils have been used as a hydraulic oil, for example, for various hydraulic apparatuses such as a construction machine, an injection molding machine, a pressing machine, a crane, a machining center, and the like. An ordinary lubricating oil has a bulk modulus that is not so high, and thus has a problem that it is difficult to exhibit sufficiently the function as a pressure transmission medium thereof. Accordingly, a lubricating oil having a high bulk modulus, which is capable of being favorably used as a hydraulic oil, has been demanded. There is a correlation between the bulk modulus and the density of a lubricating oil, and a lubricating oil having a higher density can be expected to have a higher bulk modulus.
For example, PTLs 1 and 2 propose, as a hydraulic oil having a high bulk modulus, a hydraulic oil containing an ester or ether having two or more aromatic rings as a base oil.
However, the hydraulic oils described in PTLs 1 and 2 have a high bulk modulus, but have a low viscosity index, have room for improvement in the capability as a lubricating oil, and have insufficient balance between the capability as a pressure transmission medium and the capability as a lubricating oil.
In view of the problem, for example, PTL 3 describes a lubricating oil composition prepared by blending a base oil containing an ester, an ether, or the like having two or more aromatic rings, with a poly(meth)acrylate having a mass average molecular weight of 50,000 or less, which is intended to provide a lubricating oil composition having a high density (i.e., a high bulk modulus) and a high viscosity index.